This invention relates to a spectrum-shape discriminator and more particularly to a spectrum-shape discriminating on/off target indicator when a signal simultaneously meets prescribed conditions on power level and spectrum shape.
The apparatus of this invention is an adaptive threshold which indicates if a signal simultaneously meets prescribed conditions on power level and spectrum shape. The invention yields a DC logic output only if the input spectrum has an equivalent "noise bandwidth" equal to, or less than, the maximum width expected for desired signals even through the power level meets the prescribed conditions. A DC logic output is obtained only for input signals which have a signal-to-noise ratio equal to or higher than a preset minimum level though the signal "noise bandwidth" meets the prescribed conditions.
One conventional method to determine signal power level is to measure the automatic gain control (AGC) voltage developed within a receiver. This AGC voltage is compared with a reference voltage in a voltage comparator to establish the minimum acceptable signal-to-noise ratio. Another method is to compare the DC voltage out of the detector of a delayed AGC receiver with a reference voltage in a comparator to obtain a DC logic indication of acceptable signal-to-noise ratio. Spectrum shape is usually determined by the relative DC levels out of a number of equal bandwidth, contiguous passband filters followed by detectors. When AGC, or a limiter, is used in the receiver ahead of the contiguous filters, their constant total-power properties give the system some capability to detect changes in spectral density due to changes in power level or spectral shape.
The advantages of the apparatus of the invention are that the signal power level and spectrum width are simultaneously measured. Unlike the AGC voltage comparison method, this invention will not yield a threshold crossing on undesirable input spectrums, such as broadband, high-level noise. This invention has superior capability to resolve spectrum width over the limiter or AGC and multiple filter bank method. Even when the bandwidth of each filter in the multiple filter bank is a fraction of the "accept" bandwidth of this invention, there are combinations of power level and spectrum width which make the indication ambiguous. A wide spectrum begins to cross only a few thresholds because the power density is reduced by the limiter on AGC to nearly the density obtained on thermal noise. But, because only a few thresholds are crossed, the logic indicates that a narrow spectrum, low level signal is present and accepts this undesirable signal. This does not occur with the present invention. If the spectrum is too wide, it is treated as thermal noise and is rejected. This property comes about because the thresholds in this invention react to the differences in power density between the "accept" bandwidth and the "reject" adjacent filters. The new method is far more sensitive to indicating power level and spectrum width than the filter bank method, for an equal number of filters.